(a) Field of the Invention
The present invention relates to a direct current to direct current (DC-DC) converter system of an electric vehicle and a control method thereof that improves efficiency of the system by controlling an output voltage of the DC-DC converter.
(b) Description of the Related Art
Enhanced fuel consumption has been a growing concern among many individuals along with strengthened exhaust gas regulations. As such, electric vehicles that include at least one electrically driven motor have begun to be manufactured as an alternative to the traditional fuel combustion engine. Such electric vehicles, which are also called an environmentally-friendly vehicles, include fuel cell vehicles, hybrid electric vehicles, plug-in electric vehicles, and pure electric vehicles, which generates driving torque by using electric power system with a high voltage and a large current. Such an electric vehicle typically includes a motor to generate a driving torque, an inverter to control driving of the motor, and a DC-DC converter for transforming the battery voltage of about 350V to 450V and supplying the transformed voltage to the inverter.
Typically, an output voltage of the DC-DC converter is set in accordance with a driving condition in which the inverter operates at its maximum required voltage. That is, such a DC-DC converter always outputs the maximum required voltage of the inverter. However, the voltage required by the drive motor may vary depending on driving conditions, and thus, by setting the output voltage of the DC-DC converter as the maximum required voltage, an unnecessary power loss may be incurred. In this case, unnecessary loss of power stored in the battery voltage may deteriorate the efficiency of the electrical system of the vehicle, resulting in deterioration of fuel efficiency.
FIG. 5 is a graph illustrating power loss at the inverter with respect to the output voltage of the DC-DC converter of a conventional electric vehicle. Referring to FIG. 5, when the voltage output from the DC-DC converter to the inverter becomes larger, the power loss at the inverter also becomes larger. By comparing a first value when the output voltage of the DC-DC converter is set to the maximum required voltage of the inverter with a second value when the output voltage of the DC-DC converter is set to the minimum required voltage, it will be understood that the power loss at the inverter is higher when the DC-DC converter is set to the maximum required voltage of the inverter.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.